Polysuccinimide is useful as a precursor for the preparation of poly(aspartic acid). Both polysuccinimide and poly(aspartic acid) are useful as scale inhibitors, detergents, pigment and mineral dispersants, additives for fertilizers, and corrosion and scale inhibitors in boilers and cooling towers. Several processes are known for the production of polysuccinimide. However, the known methods for the synthesis of polysuccinimide are complex, expensive, or require excessively long process times. The expense of these processes may be a result of the starting material used, the process conditions, or the recovery steps.
One method for the synthesis of anhydropoly(aspartic acid), also known as polysuccinimide, is disclosed in U.S. Pat. No. 5,057,597 to Koskan. This process uses aspartic acid as a starting material, and requires fiuidizing the aspartic acid by agitation in a nitrogen atmosphere at a temperature of at least 180.degree. C. for three to six hours. The resultant anhydropoly(amino acid) is then hydrolyzed to poly(aspartic acid).
U.S. Pat. No. 4,590,260 to Harada, et al. discloses a process for preparing copoly(amino acids) by admixing at least one species of amino acid with at least one compound selected from the group consisting of an ammonium salt of malic, maleic or fumaric acid, or that of the monoamide thereof; or malic, maleic or fumaric acid monoamide or diamide; and subjecting the mixture to thermal polycondensation and then hydrolysis.
U.S. Pat. No. 4,696,981 to Harada, et al. discloses a process for preparing homopoly(amino acids) by the application of microwaves to a mixture of one or more kinds of monoammonium, diammonium, monoamide, diamide or monoamideammonium salts of malic acid and/or maleic acid and/or fumaric acid. The resulting poly(amino acid) imide is then hydrolyzed to form the corresponding poly(amino acid).
Japanese Patent Number 44-09394B to Kobayashi, et al. discloses a process for preparing polysuccinimide by the polymerization of maleimide in the presence of a basic catalyst and a vinyl polymerization inhibitor.
U.S. Pat. No. 3,923,751 to Iwashita, et al. discloses a process for preparing poly(amino acids) by polymerizing amino acid halide salt in a non-polar organic solvent in the absence of a base. The amino acid halide salts include amino acid chloride hydrochlorides and amino acid bromide hydrobromides.
Japanese Patent Number 52-08873B to Ariyoshi, et al. discloses a process for preparing poly(aspartic acid) by polymerizing the hydrohalic salts of aspartic acid anhydride, optionally in a non-polar organic solvent.
A method for the synthesis of poly(aspartic acid) from the ammonium salt of maleic acid is disclosed in U.S. Pat. No. 4,839,461 to Boehmke. This process is carried out by heating maleic acid or maleic anhydride, which changes into maleic acid while being heated with water, and ammonia to give a dry melt of the monoammonium to biammonium salts which, if the temperature is raised is converted at 120.degree. to 150.degree. C. into poly(aspartic acid). The maleic acid and ammonia are reacted in a molar ratio of 1:1-1.5. The mixture is then heated to 120.degree.-150.degree. C. and the resulting solution of ammonium salt of maleic acid is evaporated, leaving a crystal mash. The crystal mash is then melted, during which time the waters of condensation and crystallization distill off. A porous mass of poly(aspartic acid) results. The entire process requires six to eight hours to complete.
These known methods for the synthesis of polysuccinimide are time consuming, complex, expensive and inefficient.
It is an object of this invention to provide a process for producing polysuccinimide by the thermal polymerization of maleamic acid.